This invention relates to microchip designs for the electrokinetic manipulation of fluidic chemical and biological materials. More specifically, this invention provides a microchip device which utilizes electrokinetic forces for the transport of materials through microchannels. The microchip device of this invention includes a membrane between adjacent or intersecting microchannels for passing ionic current while inhibiting bulk fluid flow.
In order to facilitate the development of the biological and chemical sciences, microchip technologies are increasingly utilized to perform traditional chemical laboratory functions within a controlled microfabricated environment. These xe2x80x9con-chipxe2x80x9d laboratories facilitate the precise transport and analysis of fluidic chemical and biological materials. Specifically, the decreased dimensions of the microchip devices provide integration of electronic and chemical processing technology while simultaneously yielding increased speed in analysis, reduction in reagent and/or sample consumption, and improved control in the automation of material manipulations.
Microfabricated devices that integrate chemical reactions with rapid analysis time have significant applications for high-throughput drug screening, automated analysis, and clinical chemistry. Microchips are characterized by reduced analysis time and reagent consumption, ease of automation, and valveless fluid control of sub-nanoliter volumes. A variety of electrically driven separations have been performed within microchannel networks. Microchips have also been developed for controlling chemical reactions, including arrays for solid-phase chemistry, reaction wells for polymerase chain reactions, channels with immobilized enzymes for flow injection analysis, and manifolds for homogenous enzyme assays. A microfluidic device using electrokinetic mixing of the organic solvents and reagents for the formation of an azo dye has been demonstrated.
The ability to design and machine channel manifolds with low-volume connections renders microchips suitable for combining several steps of an analytical process on one device. Microchips that combine chemical reactions with the speed and scale of CE analysis have been demonstrated for pre- and post-separation reactions, for DNA restriction digests with fragment sizing, and for cell lysis, multiplex PCR amplification and electrophoretic sizing.
Presently, chemical and biological materials are transported on microchips by way of electrokinetic techniques or an external pumping apparatus. The use of an external pumping apparatus is disfavored, however, because it demands additional hardware that is bulky and difficult to interface with the microchips. On the other hand, electrokinetic techniques, i.e., electroosmotically induced fluid flow or electrophoretic migration of ions, are the preferred methods of manipulating biological and chemical materials on microchip devices.
Electroosmosis is the bulk flow of fluid due to the combined effects of an electrical double layer in the presence of an axial electrical field. See, e.g., C. L. Rice and R. Whitehead, xe2x80x9cElectrokinetic Flow in a Narrow Cylindrical Capillaryxe2x80x9d, J. of Phys. Chem. (1965). The high density of ions in the diffuse region of the double layer are pulled electrostatically by the electric field along the walls of the channel. The layer of ions acts like a sleeve that is being pulled along the wall which adds momentum to the fluid by viscous drag. Under steady state conditions, which are reached in a microsecond timescale for the dimensions discussed herein, all fluid which is farther from the wall than the diffuse region are traveling at the same velocity. For example, water at pH 8 in a glass microchannel would travel at a velocity of ≅1 cm/s with an electric field strength of ≅1 kV/cm. Electrophoresis is the velocity imparted to an ion insolution when exposed to an electric field. The velocity of the ion is determined by the charge of the ion, the electric field strength, the viscosity of the solvent and the hydrodynamic radius of the ion. The direction of the ion movement depends on the direction of the electric field vector and the polarity of the charge on the ion. Electrophoresis necessarily only transports charged species. Electroosmosis imparts a velocity to all ions and neutral species. Under conditions where both electroosmosis and electrophoresis are operative, the net velocity of an ion will be the vector sum of the electroosmotic and electrophoretic velocities.
Electrokinetic transport mechanisms have been highly effective for demonstrating a number of highly useful experiments as identified above. A deficiency of presently demonstrated devices is the inability to make electrical contacts directly within microchannels. Efforts have been made to make such electrical contacts using a metal film that is photolithographically deposited onto a glass substrate so as to make contact with the fluidic microchannels. Such electrodes produce electrolysis products, most notably, oxygen and hydrogen gas from water, in all cases except under very limited conditions. The formation of a gas bubble can quickly separate the fluid in a microchannel and produces a nonconducting region which hinders the electrokinetic transport mechanisms.
The present invention provides a microfabricated device for liquid phase chemical and biological analysis. The device includes a substrate microfabricated with a series of channels and reservoirs. In accordance with this invention at least two of the microfabricated channels either intersect or are in close proximity to each other but do not overlap. A bridging membrane is created in one of the intersecting channels or between the two adjacent channels. The bridging membrane permits ionic current flow or gas transport while inhibiting bulk fluid flow therethrough. Reservoirs are formed in fluidic communication with the etched channels and are electrically connected with a high voltage power source to provide an electrical potential for electrokinetically driving and/or injecting materials from the reservoirs into the channels.
An object of the present invention is to provide a microfabricated device for performing sample loading and injection procedures that minimize electrochemically generated products in the transported sample.
Another object of the present invention is to provide reagent processing of electrokinetically driven products in a microfabricated device having a region that is uninfluenced by an electric field.
A further object of the present invention is to provide a microfabricated device which enables the transport of fluidic chemical and biological materials by electroosmotic forces into a region uninfluenced by an electric field.
Another object of the present invention is to provide a microfabricated device capable of concentrating ionic species.
A further object of the present invention is to provide a microfabricated device for separating or purifying a sample material.
Another object of the present invention is to provide a microfabricated device to facilitate the removal of electrochemically generated gas species.
A still further object of the present invention is to provide a microfabricated device to generate positive or negative pressure to facilitate hydraulic transport of gases or liquids.
Another object of the present invention is to provide a microfabricated device to effect valving in microfluidic structures.